Looper controlled rolling mill

ABSTRACT

A Rod and Bar Merchant mill is described wherein the output signals from the loopers measuring the loop depth between stands is amplified by increasing factors with increased loop depth beyond a predetermined amount. The amplified signals from the loopers then are utilized to control the speed regulator supervising the motor drive of the adjacent stand to return the loop to the desired configuration. For decreasing loop depths, a constant amplification factor preferably is utilized to adjust the signals prior to feeding the signals to the speed regulators of the adjacent stands.

United States Patent 1 1 1 1 1,304 Gorker May 21, 1974 [5 LOOPER CONTROLLED ROLLING MILL 2,512,372 6/1950 Pakala 226/42 ux [75] n n o org E. C e a y 3,550,414 12/1970 List 72/16 X Primary ExaminerMilton S, Mehr [73] Assignee: General Electric Company, Salem, Attorney, Agent, or Firm-Arnold E. Renner; Harold Va. H. Green, Jr. [22] Flled: Sept. 20, 1972 ABSTRACT [211 PP 290,613 A Rod and Bar Merchant mill is described wherein the output signals from the loopers measuring the loop 52 us. c1 72/17, 226/42, 72/29, depth between Stands is amplified y increasing 72/227 tors with increased loop depth beyond a predeter- 51] Im. Cl B2lb 37/04 mined amount The amplified Signals from the loopers 581 Field of Search 72/9, 10, 17; 226/42 then are utilized to control the Speed regulator p vising the motor drive of the adjacent stand to return 5 References Cited the loop to the desired configuration. For decreasing UNITED STATES PATENTS loop depths, a constant amplification factor preferably I is utilized to adjust the signals prior to feeding the sigy s '7 nals to the speed regulators of the adjacent stands. 312401411 3/1966 Zarleng 226/42 13 Claims, 12 Drawing Figures R RS2 33 fs k s I f V WR X \11 W X WR DMI X R 1.1 R L2 2 OM OM s m SI 3 s2 SPEED SPEED REGULATOR /FG| REGULATOR FUNCTION FUNCTION GENERATOR F62 GENERATOR SPIN J 5172 SIGNAL PROCESSING L PRg E S siNG cmcun CIRCUIT PATENTEDIAYZI am I 3.811.304

SHEEI 1 0f 4 I R FIG.| /RS| RS2 I f I i lsz SPEED rSRZ SPEED REGULATOR FUNCTION FUNCTION REGULATOR GENERATOR F62 GENERATOR SP1 s72 SIGNAL PROCESSING L Pfig E s l NG CIRCUIT CIRCUIT f: o 2 I FIG-2 s I I n ..1 KC A D I a] 8 l5-- B a O: a 5 A2 8 7- 10-0 A l O H 5 R H REFERENCE HEIGHT 0F LOOPER LOOP STORAGE (L-S) XIOO.

PATENTEDHAY 2 1 19 74 SHEET 2 BF 4 TO DMI MOTOR SPEED REGULATOR FIG. 3

FROM Ll iii'l sl'olT DIFFERENTIAL VOLTAGE TO SI (L-RHVOLTS) FIGS iATENTEDMY 21 I914 381L304 SHEET 3 0F 4 FIG.6 3s

LOOPER SIGNAL REFERENCE SIGNAL LOOPER SIGNAL ,REFERENCE SIGNAL TO SR 44 I LOOPER SIGNAL 7 PATENTED m 2 1 I974 SHEET h 0F 4 TO SR REFERENCE L FROM LOOPER REFERENCE SIGNAL in REFERENCE TO SR TO SR To SR PROCESS LOOPER COMPUTER 1 LOOPER CONTROLLED ROLLING MILL This invention relates to a rolling mill having looper control to maintain constant tension in rolled material and, in particular, to a rolling mill wherein the signal from the looper is adjusted to be at least proportional to the increase in material storage above a predetermined amount.

In rolling metal, particularly Rod and Bar Merchant mills utilized to produce a large variety of shaped prodacts, a predetermined surplus of material often is provided between rolling stands to form loops extending from the mill pass line. Because an increase or decrease in surplus material can respectively result in undesired increases in the bending stress or tension within the material being rolled, looper control systems conventionally are positioned between stands to generate a signal for adjustment of the speed of an adjacent rolling stand upon a measured change in loop depth. While such looper control systems heretofore have functioned.

quite suitably when the loop depth decreased between stands, the dynamic response of the looper control system is poor when the depth of the loop increases above the desired predetermined minimum.

It is therefore an object of this invention to provide a rolling mill having a highly accurate, looper control system wherein the dynamic response to increased loop depth is significantly enhanced without adversely affecting the system response to decreased loop depth.

It is also an object of this invention to provide a rolling mill having a looper control system wherein dynamic response to increased or decreased loop depth is approximately equal.

It is a further object of this invention to provide a looper control system wherein rapid correction of increased loop depth is achieved economically and without significant change in existing looper control design.

These and other objects of this invention generally are achieved by positioning a function generator between the looper control sensing means and the speed regulator for the adjacent stand drive motor to produce a gain in the signal from the looper sensing means which gain varies at least linearly with the increase in loop storage between stands. Thus. a rolling mill in accordance with this invention would include a plurality of tandem rolling stands for producing incremental reduction in the thickness of an elongated member threaded through the stands and means for driving each of the stands at a predetermined speed to maintain excess material between the stands in the form of loops. Looper means are provided for sensing the configuration of the material between stands and the speed of the stand adjacent the looper means is adjusted by suitable means in response to the signal from the looper means to maintain the material between stands in a predetermined configuration. In accordance with this invention, means also are provided for determining from the looper sensing means the type deviation of the member from the preselected configuration and the signal from the looper means is adjusted by suitable means to generate a speed correction signal which signal increases at least linearly with increased storage of the material between stands. Preferably. the increase is accomplished by amplifying the signal from the looper means by a first factor upon sensing decreased storage of material between stands and amplifying the signal from the looper means by a greater factor upon sensing increased storage of material between stands.

Although this invention is described with particularity in the appended claims, a more complete understanding of the invention may be obtained from the following detailed description of a rolling mill controlled in accordance with this invention when taken in conjunction with the appended drawings wherein:

FIG. I is a schematic illustration of a rolling mill having a looper control system in accordance with this invention,

FIG. 2 is a graph showing the variation in loop depth and output voltage from the looper with loop storage between rolling stands,

FIG. 3 is a block diagram of a looper control system for signal adjustment in accordance with this invention,

FIG. 4 is a schematic diagram of a function generator suitable for use in this invention,

FIG. 5 is a graph illustrating the adjusted signal generated by the circuit of FIG. 4 with increased depth of loop between stands,

FIG. 6 is a schematic diagram'of an alternate function generator circuit suitable for utilization in the circuit of FIG. 3,

- FIG. 7 is a schematic diagram of the circuit illustrated in FIG. 3 wherein voltage variable resistors are utilized as the function generator,

FIG. 8 is a schematic diagram ofa looper control circuit wherein the function generator is serially connected between the amplifiers of the looper control system, I

FIG. 9 is a schematic diagram of a looper control circuit utilizing voltage variable resistors in a serially connected function generator, 7 I

FIG. 10 is a block diagram of an alternate looper control system in accordance with this invention,

FIG. 11 is a schematic diagram of a looper control system in accordance with FIG. 10, and

FIG. 12 is a schematic diagram of an alternate looper control system in accordance with the block diagram illustrated in FIG. 10. j

A rolling mill l0 controlled in accordance with this invention is illustrated in FIG. I and generally comprises a plurality of tandem rolling stands, illustrated as three rolling stands RSI-RS3, for incrementally reducing the thickness of bar B as the bar passes through the stands. The bar, in conventional fashion, is looped between stands, i.e., the total length TL of the bar between rollers X supporting the bar is greater than the horizontal span S between rollers, to produce a nonlinear configuration in the bar (shown as downward extending loops in FIG. 1). It will be appreciated, however, that the loops also could extend upwardor in a horizontal direction dependent upon the desired operating mode of the mill. A workroll WR at each stand is driven by an associated adjustable speed drive motor DM and the configuration of the loop between stands is sensed by a looper L to generate a signal which is employed to adjust the speed of the adjacent stands around an anchor stand, i.e., to adjust the speed of rolling stands RS1 and RS3 relative to rolling stand RS2.

error signals indicative of loops greater than the predetermined depth. The output signal from function generator FGl then is fed to a conventional signal processing circuit SP1 (i.e., a proportional and integral control circuit) before being forwarded to the speed regulator SR1 controlling the speed of the adjacent upstream motor DMl. In a similar fashion, the output signal from looper L2 on the downstream side of rolling stand RS2 is fed to a summing amplifier S2 to produce an error signal indicative of depth variations in the loop between stands RS2 and RS3. This error signal again is fed through a function generator F02 and a signal proc'essing circuit SP2 to speed regulator SR2 controlling the speed of downstream stand RS3.

Prior to this invention, the output signal from sum ming amplifiers S1 and S2 have been fed directly to the respective signal processing circuits, i.e., SP1 and SP2, for speed regulation purposes. However, as can be seen from curve A in FIG. 2, the loop depth H and the out put signal V from the loopers has been found not to vary linearly with loop storage between stands. Thus, for decreasing storage of bar B between stands below apredetermined depth H,,, the output signal from the looper decreases approximately exponentially (as shown by curve portion Al) while the output signal from the looper for loop storage above the predetermined depth increases at a rate less than a linear relationship between the variables (as shown by curved portion A2 in FIG. 2). To correct for the reduced response in output signal from the looper with increasing loop storage, function generator F0 is inserted into the speed control circuit to amplify the error signal from the looper for increased loop depth by gradually increasing factors with increased loop storage to produce at least a linear response in the output signal from the looper control with depth. as shown by curve B in H0. 2. A better dynamic response, however, is obtained when the output signal from the looper is altered by an amount to produce an exponential increase in the looper signal with increased loop storage, as shown in curve C of FIG. 2. Because the output signal from the looper varies in an exponential fashion for decreased loop storage, the looper signal preferably is utilized without alteration to adjust the speeds of the adjacent stands in order to obtain a symmetrical response for the control system. To achieve a symmetrical response, some decrease in the outputsignal fromthe loopers may be required for decreased storage should the gain produced by the function generators F61 and F62 not be sufficiently high to achieve an exponential increase in output signal with increased storage of the bar between stands. The function generators, however, should produce an output signal from the looper for increased loop depth defined approximately by the formula:

V (adjusted) V (ll Bl wherein V is the output signal from the looper and a and B are constants, typically in the order of tenths and hundreths.

A block diagram of a circuit suitable for producing an exponentially increasing output signal with increased loop storage is illustrated in FIG. 3 and generally includes a first summing amplifier S1 into which is fed the output signal from looper Ll and'a reference signal R proportional to the looper output with a desired depth of loop between stands. The output signal from the summing amplifier then is fed to function generator FGl having a gain which varies approximately proportional to the increase in the output signal from the summing amplifier. For those error signals having a polarity indicative of a decrease in loop storage, the function generator preferably will have a constant gain. After adjustment of the error signal by the function generator, the signal is fed to signal processing circuit SP1 which includes a proportional control PC and an integral control IC connected in parallel with the output signals from the control circuits being fed to summing amplifier SA to produce the desired correction signal for the motor speed regulator SR. Useof a proportional control and an integral control in parallel generally is a well-known technique in the control art and is utilized to produce a rapid correction signal, i.e., by way of the proportional control circuit, for-rapid adjustment of the loop depth and a slower, more comprehensive correction signal, i.e., by way of the integral control circuit, to adjust the loop depth by a factor proportional to loop depth variations over a prolonged period. Because the integral control circuit IC gradually approaches a setting reducing the loop variation to approximately zero, a normally closed hold contact C is inserted in series between the looper and the integral control to maintain the integral control at the last fixed setting as the end of bar B passes the looper.

A circuit with function generator F6 in the feed back path of summing amplifier 51 is illustrated in FIG. 4 wherein two serially connected amplifiers S1 and SA again serve to adjust the signal from looper Ll prior to feeding the signal to the motor speed regulator SR to improve thedynamic response of the stand to loop depth variations. As in FIG. 3, the signal from the looper and reference signal R are fed to summing amplifier S1 through resistors R1 and R2 to produce a differential output signal from the amplifier which differential signal is fed to a phase advance circuit 21 to adjust the phase of the output signal while the gain of the v first amplifier circuit is adjusted by the setting of potentiometer 22. To obtain an exponentially increasing signal with increased loop storage, the output signal from summing amplifier S1 is fed back through three paths, two of which exhibit impedance values which vary with the magnitude of the output signal from the amplifier. The first variable impedance path generally includes a pair of serially connected resistors 23 and 24 extending from the output terminal of the amplifier to the input terminal while the midpoint of the resistors is connected to ground through three serially connected diodes Dl-D3. Similarly, the output signal from the summing amplifier is fed back to the input terminal of the amplifier through a second pair of serially connected resistors 26 and 27 with the midpoint of the resistors being connected to ground through diode D4 and zener diode 28. A third resistor 30 also is connected in parallel across resistors 26 and 27 to provide a third feed back path (i.e., the constant impedance path) for the output signal from the amplifier.

When the output signal from the looper is greater than the reference signal, i.e., when the depth of the loop increases beyond thepredetermined desired configuration, a positive voltage is produced by summing amplifier S1, and upon the positive voltage increasing above the breakdown levels of diodes Dl-D3, i.e., at approximately 1 volt, the lower feed back path through resistors 23 and 24 is by-passed by the conducting diode to produce a first break point A in the output signal 32 from the summing amplifier illustrated in FIG. 5. Should the depth of the loop continue to increase, the threshhold level of zener diode 28 will be exceeded and the feed back path through resistors 26 and 27 is terminated by conduction to ground through the zener diode to providea second break point, i.e., point B, in the output signal from the amplifier. The incremental gain of the amplifier then is determined solely by resistor 30 and the input impedance of the amplifier. After adjustment of the phase of the output signal from the summing amplifier in phase advance circuit 21 and trimming of the gain by the setting of potentiometer 22, the adjusted output signal from the summing amplifier is fed through hold contacts C and input resistor 33 to amplifier SA. Amplifier SA, in known conventional fashion, functions as both a proportional control and an integral control with capacitor 34 providing the charge storage for long term correction of the feed back circuit while resistor 35 provides the more rapid proportional response to the output signal. A re-set switch 36 and a resistor 37 are connected in parallel with capacitor 34 to permit discharge of the capacitor when desired.

When the depth of the loop decreases, the output signal from summing amplifier S1 is negative in polarity rendering diodes D1, D2, D3 and D4 non-conductive. The amplifier gain then is determined by the parallel combination of the three resistive paths. Because the impedance within the feed back path does not vary with the magnitude of the negative output signal from the amplifier, a straight line relationship (illustrated by line C of output signal 32 in FIG. 5) is obtained between the output signal from amplifier SI and loop storage between stands.

While the circuit illustrated in FIG. 4 employs two break points, i.e., points A and B, to produce an exponential increase in the output signal from the summing amplifier, it will be appreciated that additional break points can be achieved merely by increasing the number of sequentially by-passed degenerative parallel feed back circuits employed with the amplifier. For example, the feed back circuitry could include four feed back paths (as shown in FIG. 6) by using a zener diode 38 having a higher positive breakdown voltage than zener diode 28 to remove the additional feed back path formed by resistors 39 and 40 with increased output voltage from amplifier S]. The circuit otherwise is identical to the circuit of FIG. 4. Thus, as the output signal from the summing amplifier 81 increases in a positive direction, the feed back path through resistors 27 and 28 will be by-passed to ground by conduction of zener diode 28 while a further increase in the output signal from the summing amplifier will result in conduction of zener diode 38 to by-pass the feed back path through resistors 39 and 40 to ground to add an additional break point in the curve of FIG. 5. Diode D5 serves to block current flow through zener diode 38 when the output signal from the summing amplifier increases in a negative direction.

It is also possible to utilize materials, such as thyrite resistors, exhibiting a conductivity variation with voltage (or current) to alter the impedance feed back circuit in the summing amplifier. One such circuit is illustrated in FIG. 7 wherein a thyrite resistor 41 is substituted for zener diode 28 in the feed back circuit of the operational amplifier depicted in FIG. 4. Thyrite resistors are well-known in the art and are characterized by a resistance which decreases as the applied voltage increases and can be utilized with operational amplifiers to generate a voltage voltage E E wherein n is an integer between 2 and 3.5. Other than the substitution of the thyrite resistor for the zener diode, the circuit of FIG. 7 is identical in structure and operation to the circuit of FIG. 4. Thus, as the output voltage from the summing amplifier increases in a positive direction, diodes Dl-D3 will initially conduct to by-pass the feed back path between resistors 23 and 24 whereafter the output signal in the amplifier is fed backonly through the parallel feed back paths consisting of resistor 30 and series resistors 26 and 27. As the output voltage from the summing amplifier further increases (indicating an increase in the difference between the output signal from looper L measuring loop depth and the reference signal R), the resistance of thyrite resistor 41 will gradually decrease to by-pass the feed back current to ground. The output signal from the summing amplifier therefore will tend to increase exponentially as shown in the curve of FIG. 5. Because increased resistance is desirable in the degenerative path of summing amplifier S1 with increased output voltage from the amplifier produced by an increase in the measured depth of the loop between stands, it will be appreciated that impedance devices exhibiting an increase in resistance with voltage also could be utilized in the series feed back path of the summing amplifier to obtain an exponential increase in the output signal with increased loop storage between mill stands.

As is illustrated in FIG. 8, the function generator for altering the output signal from the looper also can be positioned serially between summing amplifier S1 and proportional and integral control amplifier SA to produce the dynamic response desired from the looper with increasing storage of bar B between stands. The output signal from looper L and the looper reference signal again are fed to the input of summing amplifier S1 which generates an output signal proportional to the difference therebetween. The newvork forcompensating for the predominant lag of the speed regulator is placed in the feed back of the summing amplifier and generally includes a resistor 43 and a capacitor 44 connecting to ground the mid-point of serially connected feed back resistors 45 and 46. The output signal from the summing amplifier is fed through hold contacts C to function generator FG which consists of three parallel resistor paths, i.e., resistors 47, 48 and 49, having zener diodes 50 and 51 with different breakdown potentials in the paths of two of the resistors. Conventional diode 52 also is connected in series with the resistor paths containing the zener diodes to block current flow through those paths when the output signal from summing amplifier S1 is a negative potential indicating a decreased storage in the loop depth between stands. Thus, for a negative output from summing amplifier S1 (and for positive voltages output less than the I breakdown voltage of the zener diodes), the current and proportionalfeed back circuitry. of the amplifier before being forwarded to the speed regulator SR controlling the drive of the adjacent stand. As the output from summing'amplifier S1 increases in a positive direction, zener diode 50 breaks down to add resistor 48 to the current conducting path between amplifiers thereby decreasing the attenuation of the output signal from the summing amplifier. Similarly, as the voltage output from summing amplifier 51 increases still further, zener diode 51 breaks down to add an additional resistor, i.e., resistor 49, in parallel with the conducting paths between amplifiers to further reduce the attentuation of the output signal from the summing amplifier. The dual amplifiers thus produce a substantially exponential increase in output signal with increased loop storage between stands. The output signal from the circuit also exhibits a substantially uniform gain, due to the presence of blocking diode 52 in the paths of zener diodes 50 and 51, for decreasing loop storage between stands, i.e., for negative going outputs from summing amplifier 81.

It is also possible to substitute a thyrite resistor 54 for the zener diodes between summing amplifier S1 and the integral and proportional amplifier SA, as is shown in FIG. 9, to obtain an exponential increase with increased output'from the summing amplifier. Other than the substitution of the thyrite resistors for the zener diodes, the circuit of FIG. 9 is identical in structure and operation to the circuit of FIG. 8. Thus, as the output signal from summing amplifier 81 increases in a positive direction, the resistance of thyrite resistor 54 decreases to lower the attenuation in the output signal from the summing amplifiers produced by the impedance network between amplifiers. Diode 52 again serves to block negative voltages from the summing amplifier from passing through the thyrite resistor when the depth of the loop decreases between stands to assure a constant gain for decreased loop storage.

While the function generator heretofore has been described as being positioned between the summing amplifier SL and the proportional and integral control amplifier SA, it is also possible to place the function generator only in series with a proportional control amplifier 54 as is shown in FIG. 10. Thus, rather than utilizing a single amplifier to produce both a proportional and integral control adjustment, :1 separate integral control amplifier 55 utilized to produce a longer-term cor rection, i.e., to adjust for continuous errors, in .the output signal from summing amplifier 81 while the more rapid response to variations from the summing amplifier is achieved by proportional control amplifier 54. The output signal from looper L and reference signal R againLare fed to summing amplifier S1 and the output signal from the summing amplifier is forwarded through integral control amplifier 55 to summing amplifier j6 before being applied to motor speed regulator SR. The output signal from summing amplifier 81 also is fed tr? function generatorFG to obtain a preferably exponentially increasing signal which is fed to proportional control amplifier 54. After adjustment of the signail in the proportional control amplifier, the signal is applied lto summing amplifier 56 to be combined with the output signal from the integral control amplifier prior to being fed to motor speed regulator SR.

' connected resistors 45 and 46 (in conjunction with resistor 43 and capacitor 44 connected between the junction of the feed back resistors and ground) provides the desired phase advance in the summing amplifier circuit. The output signal from the amplifier is fed to function generator FG consisting of three parallel resistors 47, 48 and 49 with zener diodes 50 and 51 being placed within two parallel paths to successively produce conduction in the paths. The output signal from the function generator then is applied to proportional control amplifier 54 before being fed to speed regulator SR. The input signal to integral control amplifier 55, however, is obtained directly from summing amplifier 51 through hold contacts C. Feedback capacitor 34 again provides the long term integration desired for the amplifier while switch 36 and resistor 37 shunting the capacitor permit periodic discharge of the capacitor. The output signal from integral control amplifier 55 is forwarded to speed regulator SR to adjust the speed of the adjacent stand in conjunction with the output signal from proportional control amplifier 54.

in a rolling mill wherein a process computer is utilized to regulate the operation of the mill, it often is beneficial to have at least the integral control performed by the computer using known integration programs rather than utilizing a separate operational amplifier for such purpose. Such a circuit is illustrated in PK]. 12 wherein process computer PC is employed to produce the integral control signal while operational amplifier S4 is employed to produce only the proportional signal. The two signals then are combined in the speed regulator SR to adjust the speed of the drive motor of the adjacent stand. The function generator FG again is positioned in the feed back path of the summing amplifier S1 and functions identical to the operation of the function generator in FIG. 3.

While this invention has been described with respect to certain specific embodiments, it will be appreciated that many modifications and changes may be made without departing from the spirit of the invention. 1 intend, therefore, by the appended claims, to cover all such modifications and changes which fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a rolling mill characterized by a plurality of tandem stands for producing incremental reductions in the thickness of an elongated member threaded through said stands, means for driving each of said stands at an initial predetermined speed to maintain excess material between stands in the form of a loop, means for sensing the physical configuration of the material between stands and means for adjusting the speed of the mill stands to maintain the configuration of said member between stands in a preselected configuration, the improvement comprising means for determining from said configuration sensing means the type of deviation of said member from the preselected configuration and means for adjusting the output signal from said configuration sensing means to generate the speed correction signal increasing at a rate faster than the measured increase in loop depth between stands.

2. A rolling mill according to claim 1 wherein said output signal is adjusted by an amount proportional to the square of the increase in loop depth between stands.

3. A rolling mill according to Claim 1 wherein said signal adjusting means produces a signal S which varies in accordance with the formula:

wherein V is the output signal from said configuration sensing means and a and B are fractional constants.

4. In a rolling mill characterized by a plurality of tandem stands for producing incremental reductions in the thickness of an elongated member threaded through said stands, means for driving each of said stands at an initial predetermined speed, means for sensing the physical configuration of the member between stands and means for adjusting the speed of the mill stands to maintain the configuration of said member between stands in a preselected configuration, the improvement comprising means for amplifying said signal from said configuration sensing means by a first factor upon sensing decreased storage of said material between stands and means for amplifying said signal from said configuration sensing means by a factor greater than said first factor upon sensing increased storage of said material between stands.

5. A rollingmill according to claim 4'wherein said second amplification factor increases with increased storage of said material between stands.

6. A rolling mill according to claim 5 wherein said first amplification factor is constant over the entire range of decreasing storage of said material between stands.

7. A rolling mill according to claim 6 wherein said second amplification factor increases exponentially.

8. A rolling mill for incrementally reducing the thickness of an elongated member passing through said mill. said mill comprising at least two tandem rolling stands driven by respective adjustable speed drives, looper means for sensing the depth of said material between stands and for generating a signal corresponding to the variation in said looper depth, means connecting said looper means to at least one stand drive adjacent said looper means to vary the speed of said drive in response to a measured variation in the depth of said loop and function generator means disposed between said looper means and said drive means to adjust the signal from said looper means, said function generator producing a gain in the signal measured by said looper means which gain varies proportional to the depth of the loop above a predetermined amount.

9. A rolling mill according to claim 8 wherein said means connecting said looper means to said'drive includes an integral control circuit, a proportional control circuit and means for retaining at least said integral control circuit in fixed position upon passage of the tail end of said material beyond said looper means.

10. A rolling mill according to claim 9 wherein said integral control and said proportional control are parallel circuits, said function. generator being serially connected within said proportional control circuit.

11. A rolling mill according to claim 8 wherein said means connecting said looper means to said drive includes summing amplifier means for generating a signal proportional to the deviation of said looper means signal from a predetermined value and said function generator means includes means for feeding back the output signal from said summing amplifier means to an input'terminal of said summing amplifier means, said feed back means including at least two feed back paths of different impedance values, said feed back paths being switched by the level of the output signal from said summing amplifier means.

12. A rolling mill according to claim 11 further including a second amplifier connected between said summing amplifier means and said drive means, said second amplifier having at least one feed back path which varies as the integral of the output signal from said amplifier.

13. A rolling mill according to claim 8 wherein said means connecting said looper means to said drive includes two serially connected amplifiers and said function generator serially connects said amplifiers through an impedance path having an impedance which varies as a function of the output signal from the first amplifier. 

1. In a rolling mill characterized by a plurality of tandem stands for producing incremental reductions in the thickness of an elongated member threaded through said stands, means for driving each of said stands at an initial predetermined speed to maintain excess material between stands in the form of a loop, means for sensing the physical configuration of the material between stands and means for adjusting the speed of the mill stands to maintain the configuration of said member between stands in a preselected configuration, the improvement comprising means for determining from said configuration sensing means the type of deviation of said member from the preselected configuration and means for adjusting the output signal from said configuration sensing means to generate the speed correction signal increasing at a rate faster than the measured increase in loop depth between stands.
 2. A rolling mill according to claim 1 wherein said output signal is adjusted by an amount proportional to the square of the increase in loop depth between stands.
 3. A rolling mill according to Claim 1 wherein said signal adjusting means produces a signal S which varies in accordance with the formula: S V + Alpha V2+ Beta V3 wherein V is the output signal from said configuration sensing means and Alpha and Beta are fractional constants.
 4. In a rolling mill characterized by a plurality of tandem stands for producing incremental reductions in the thickness of an elongated member threaded through said stands, means for driving each of said stands at an initial predetermined speed, means for sensing the physical configuration of the member between stands and means for adjusting the speed of the mill stands to maintain the configuration of said member between stands in a preselected configuration, the improvement comprising means for amplifying said signal from said configuration sensing means by a first factor upon sensing decreased storage of said material between stands and means for amplifying said signal from said configuration sensing means by a factor greater than said first factor upon sensing increased storage of said material between stands.
 5. A rolling mill according to claim 4 wherein said second amplification factor increases with increased storage of said material between stands.
 6. A rolling mill according to claim 5 wherein said first amplification factor is constant over the entire range of decreasing storage of said material between stands.
 7. A rolling mill according to claim 6 wherein said second amplification factor increases exponentially.
 8. A rolling mill for incrementally reducing the thickness of an elongated member passing through said mill, said mill comprising at least two tandem rolling stands driven by respective adjustable speed drives, looper means for sensing the depth of said material between stands and for generating a signal corresponding to the variation in said looper depth, means connecting said looper means to at least one stand drive adjacent said looper means to vary the speed of said drive in response to a measured variation in the depth of said loop and function generator means disposed between said looper means and said drive means to adjust the signal from said looper means, said function generator producing a gain in the signal measured by said looper means which gain varies proportional to the depth of the loop above a predetermined amount.
 9. A rolling miLl according to claim 8 wherein said means connecting said looper means to said drive includes an integral control circuit, a proportional control circuit and means for retaining at least said integral control circuit in fixed position upon passage of the tail end of said material beyond said looper means.
 10. A rolling mill according to claim 9 wherein said integral control and said proportional control are parallel circuits, said function generator being serially connected within said proportional control circuit.
 11. A rolling mill according to claim 8 wherein said means connecting said looper means to said drive includes summing amplifier means for generating a signal proportional to the deviation of said looper means signal from a predetermined value and said function generator means includes means for feeding back the output signal from said summing amplifier means to an input terminal of said summing amplifier means, said feed back means including at least two feed back paths of different impedance values, said feed back paths being switched by the level of the output signal from said summing amplifier means.
 12. A rolling mill according to claim 11 further including a second amplifier connected between said summing amplifier means and said drive means, said second amplifier having at least one feed back path which varies as the integral of the output signal from said amplifier.
 13. A rolling mill according to claim 8 wherein said means connecting said looper means to said drive includes two serially connected amplifiers and said function generator serially connects said amplifiers through an impedance path having an impedance which varies as a function of the output signal from the first amplifier. 